Due to the shortage of hydrocarbon fuels the importance of silicon solar cells as an energy source has increased greatly in the last few years. The efficiency of silicon in absorbing solar radiation, however, is only about 60%, the other 40% being reflected back into the atmosphere. The amount of light reflected can be reduced by coating the silicon with a material having an index of refraction between that of silicon and air according to Fresnel's equation: ##EQU1## where n.sub.0 is the index of air, n.sub.1 is the index of the coating, and n.sub.2 is the index of the silicon.
Thus, for silicon, which has an index of about 4, the coating should have an index of about 2.0. (If the coating is applied on a slightly oxidized surface, the index may have to be adjusted.) This maximum reduction in the amount of light reflected is achieved at a coating thickness of one quarter of a wavelength.
Presently, it is known how to deposit coatings having specific indices of refraction. For example TiO.sub.2 coatings have an index of 2.5 to 2.7, Al.sub.2 O.sub.3 coatings have an index of 1.76, SiO.sub.2 coatings of 1.55, etc. In addition to the fact that the indices of refraction of these coatings are not exactly 2 (or whatever index is needed), the chemical vapor deposition, vacuum deposition, and RF sputtering techniques for applying them are very expensive.